Diffuser plate for boiler burner feed assembly

ABSTRACT

A diffuser or orifice plate is positioned across the bore of a feed pipe supplying a mixture of pressurized air and recycled flue gas to the combustion chamber of a gas burner associated with a boiler. The diffuser plate comprises inner and outer, radially spaced apart, annular baffles. The baffles define a central opening and an outer series of openings forming an annular array. Part of the gas mixture is accelerated as it passes through the restrictive outer openings and forms an annular stream. Part of the gas mixture passes through the central opening in the form of a lance-like stream. Fuel gas is injected into the annular stream immediately downstream of the diffuser plate. The resulting air/flue gas/fuel gas mixture mixes and adopts an elongated form in the combustion chamber. The burner feed assembly described contributes to maintaining the Ox content in the burner flue gas at a low level such as less than 15 ppm.

FIELD OF THE INVENTION

The present invention relates to a burner feed assembly incorporating adiffuser plate. The burner feed assembly is used in connection with theburner of a boiler, such as a boiler used in an oilfield operation forgenerating steam. The diffuser plate assists in mixing air, recycledflue gas and fuel gas being fed to the burner combustion chamber.

BACKGROUND OF THE INVENTION

Boilers are commonly used in oilfield operations. These boilers areusually provided to generate steam.

The burners in these boilers produce flue gases, which are commonlyemitted into the atmosphere. The flue gases comprise NOx compounds. Theproduction of these NOx compounds is environmentally undesirable. Theycan react to form ground-level ozone and nitrates, which can triggerrespiratory problems in humans. They also contribute to the formation ofacid rain. And nitrous oxide contributes to the formation of‘greenhouse’ gases in the atmosphere.

Governments are now issuing increasingly stringent guidelines withrespect to acceptable levels of NOx emissions, such as those produced byoilfield boiler burners.

It is well understood in the art that thorough mixing of the air andfuel being fed to a burner combustion chamber can lead to a reduction inNOx emissions.

It is also known that reducing combustion temperature in the burnerchamber can ameliorate NOx emissions.

One prior art embodiment of an oil burner feed assembly, commerciallysupplied for steam boilers, is illustrated schematically in FIGS. 1 and2. The burner feed assembly (a) comprises a diffuser plate (b) mountedwithin the downstream end of an air feed pipe (c). In use, the feed pipe(c) is connected to the combustion chamber of a boiler burner. Thediffuser plate (b) extends across the throat or bore of the feed pipe(c). The feed pipe (c) is a conduit for a stream of pressurized airissuing from a blower or fan (d). Some flue gas is recycled from aburner stack and is fed into the feed pipe bore, upstream of thediffuser plate (b). As shown, the diffuser plate (b) comprises anannular baffle (e). An oil nozzle (f) is mounted in the central opening(g) of the baffle (e), for injecting atomized oil. The baffle (e) formsa plurality of outer openings (h). The mixture of air and recycled fluegas flows through the openings (h) and mixes with the oil injected bythe nozzle (f) in the burner chamber, to produce the combustion mixture.

The flue gas is added to produce a “lean” air/flue gas/fuel gas mixturewhich burns at a moderate temperature.

One state government has recently regulated that oilfield steam boilerburner NOx emissions should be less than 15 ppm. The state of the artburners described above produce NOx emissions of about 25 ppm.

There is therefore a need for a burner feed assembly adapted to betterenable the burner to produce reduced NOx emissions, for example at aconcentration less than 15 ppm.

It is the purpose of the present invention to provide such an assembly.

SUMMARY OF THE INVENTION

The present invention relates to a modified burner feed assembly for usein the context of a fuel gas-fired boiler.

The objective of the invention is to reduce NOx emissions in flue gasesleaving the boiler burner. This objective is achieved by the combinationof:

-   -   arranging for a central air/flue gas flow that causes elongation        of the flame in the burner combustion chamber;    -   arranging for an outwardly spaced, generally annular air/flue        gas flow that surrounds the central flow; and    -   introducing fuel gas inwardly from the side into the outer gas        flow; whereby a desirable degree of mixing of the air, flue gas        and fuel gas is obtained to produce a generally homogeneous        combustion mixture and the resulting flame is elongated and is        characterized by a moderate flame temperature.

These results are achieved by the combination of using a diffuser plateof specific configuration and introducing the fuel gas inwardly,immediately downstream of the plate.

More particularly, the diffuser plate comprises:

-   -   inner and outer, radially spaced apart, annular baffles forming        a central opening and one or more arcuate openings spaced        outwardly from the central opening; and    -   means, such as radial connectors, for interconnecting the        baffles.

It is believed that the upstream surfaces of the annular baffles deflectpart of the air/flue gas mixture flow moving through the feed pipe andcause it to move through the arcuate openings, which constrict andaccelerate it, thereby inducing a venturi effect which assists inentraining the fuel gas being injected inwardly, immediately downstreamof the plate. In addition, it is believed that the lance-like flowthrough the central opening has the effect of causing the flame producedin the burner chamber to elongate, which contributes to reducing flametemperature.

In another aspect, the invention is concerned with a method forcontrolling NOx emission from a burner having a feed pipe for supplyinga flow of a mixture of air and flue gas under pressure into a burnercombustion chamber, comprising:

-   -   providing an apertured diffusion plate extending across the bore        of the feed pipe, said plate having a central opening and one or        more outwardly spaced, arcuate openings;    -   forming part of the mixture flow into an inner stream having an        elongated configuration, by passing it through the central        opening;    -   forming another part of the mixture flow into an outer stream        having a generally annular configuration by passing it through        the outer openings;    -   the area of the outer openings being selected to be restrictive        so that the mixture flow is accelerated as it passes        therethrough; and    -   injecting fuel gas inwardly into the annular stream immediately        downstream of the plate.

In an apparatus aspect, the invention is concerned with a burner feedassembly for supplying a combustion mixture to the combustion chamber ofa burner associated with a boiler, comprising: means for supplying astream of pressurized air; a feed pipe, having a bore, for connectingthe air supply means with the combustion chamber, said bore havingupstream and downstream ends; a diffuser plate, comprising inner andouter, generally annular baffles forming a central opening and at leastone outwardly spaced, arcuate opening, said diffuser plate beingpositioned transversely across the downstream end of the bore of thefeed pipe; means for recycling flue gases, produced in the combustionchamber, to the feed pipe bore upstream of the diffuser plate, formixing with the stream of air to form an air/flue gas mixture; and meansfor feeding fuel gas into the air/flue gas mixture immediatelydownstream of the diffuser plate.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified side view showing a burner feed assembly inaccordance with the prior art, said assembly being connectable with theburner of a boiler;

FIG. 2 is a simplified end view of the prior art burner feed assembly ofFIG. 1, showing the diffuser plate;

FIG. 3 is a simplified schematic side view showing a burner feedassembly in accordance with the invention, connected with the burner ofa boiler, with a fanciful burner flame shown;

FIG. 4 is a front view showing a diffuser plate in accordance with theinvention; and

FIG. 5 is a view similar to that of FIG. 4, labelled to indicate thedimensions of one embodiment, as set forth in Table 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The burner feed assembly 1 is adapted for operation with a burner 2 of asteam boiler 3.

The burner feed assembly 1 comprises means, such as a blower 4, forsupplying a stream of pressurized air 5 into the axial bore 6 of a feedpipe 7. The feed pipe 7 connects the blower 4 with the combustionchamber 19 of the burner 2.

A recycle line 8 connects the stack 9 of the burner 2 with the blower 4,for recycling some of the produced flue gas 10 into the bore 6. Therecycled flue gas 10 and air 5 mix in the bore 6 to form an air/flue gasmixture 11.

A diffuser plate 12 is mounted in the feed pipe 7 at its downstream end,so as to extend across the bore 6. The diffuser plate 12 comprises innerand outer annular baffles 13, 14. The baffles 13, 14 are interconnectedby connectors 15 and are fixed in a common plane. The baffles 13, 14 andconnectors 15 combine to define a central opening 16 and a plurality ofoutwardly spaced, arched or arcuate openings 17. The arcuate openings 17combine to form a substantially annular passageway 18.

As a result of experimentation, applicant has identified a preferredcombination of dimensions for the diffuser plate, which yieldparticularly desirable results. These dimensions are identified in FIG.5 and are set forth in the following tabulation: TABLE 1 a 23 1/16″ b18½″ c 11 2/4″ d  5½″

A fuel line 20 connects a source 2 a of fuel gas 21 with the bore 6 ofthe feed pipe 7, immediately downstream of the diffuser plate 12. Thefuel line 20 feeds fuel gas 21 inwardly into the annular stream 22 ofair/flue gas mixture 11 which issues from the arcuate passageway 18.

In operation, the blower 4 and recycle line 8 supply pressurized air 5and recycled flue gas 10 into the feed pipe bore 6, to form acylindrical stream 24 of mixture 11. Part of the mixture stream 24 flowsthrough the central opening 16 to produce a lance-like stream. Theremainder of the mixture stream 24 is accelerated as it passes throughthe restrictive arcuate openings 17 and produces a generally annularstream which entrains fuel gas 21 being fed in from the side and mixesit with the air and flue gas.

Example

A diffuser plate dimensioned in accordance with Table 1 was installed inthe bore of the feed pipe of a North American Manufacturing CompanyModel G burner connected to a steam boiler. Air was supplied to the feedpipe at 10,000 C.F.M. and 1 psig. Flue gas was recycled to the feedpipe, upstream 30 inches from the diffuser plate, at a rate of 1,000C.F.M. Fuel gas was injected from the side, 2 inches downstream of theplate, at a rate of 833 C.F.M.

The flue gas emitted by the burner was tested using a Teco™Chemiluminescent Analyzer and was found to contain 12 ppm NOx.

1. A burner feed assembly for supplying a combustion mixture to thecombustion chamber of a burner associated with a boiler, comprising:means for supplying a stream of pressurized air; a feed pipe, having abore, for connecting the air supply means with the combustion chamber,said bore having upstream and downstream ends; a diffuser plate,comprising inner and outer, generally annular baffles forming a centralopening and at least one outwardly spaced, arcuate opening, saiddiffuser plate being positioned transversely across the downstream endof the bore of the feed pipe; means for recycling flue gases, producedin the combustion chamber, to the feed pipe bore upstream of thediffuser plate, for mixing with the stream of air to form an air/fluegas mixture; and means for feeding fuel gas into the air/flue gasmixture immediately downstream of the diffuser plate.
 2. The burner feedassembly as set forth in claim 1 comprising radial connectorsinterconnecting the inner and outer annular baffles, said baffles andconnectors combining to define a plurality of arcuate, outwardly spacedopenings, collectively forming a substantially annular passageway.
 3. Amethod for controlling NOx emission from a burner having a feed pipe forsupplying a flow of a mixture of air and flue gas under pressure into aburner combustion chamber, comprising: providing an apertured diffusionplate extending across the bore of the feed pipe, said plate having acentral opening and one or more outwardly spaced, arcuate openings;forming part of the mixture flow into an inner stream having anelongated configuration, by passing it through the central opening;forming another part of the mixture flow into an outer stream having agenerally annular configuration by passing it through the outeropenings; the area of the outer openings being selected to berestrictive so that the mixture flow is accelerated as it passestherethrough; and injecting fuel gas inwardly into the annular streamimmediately downstream of the plate.